⁌ ULY ⁍added a comment 5 years agoWATCHFUL EYE is much more common: https://books.google.com/ngrams/graph?content=keep+a+wary+eye%2Ckeep+a+watchful+eye&year_start=1800&year_end=2008&corpus=15&smoothing=3&share=&direct_url=t1%3B%2Ckeep%20a%20wary%20eye%3B%2Cc0%3B.t1%3B%2Ckeep%20a%20watchful%20eye%3B%2Cc0